Device for converting an electric current

ABSTRACT

A device for converting an electrical current includes at least one phase module with an AC voltage connection and at least one DC voltage connection, a phase module branch disposed between each DC voltage connection and the AC voltage connection and each phase module branch having a series circuit of submodules, each of which has an energy accumulator and at least one power semiconductor and closed-loop control means for regulating the device. The device can regulate circulating currents in a targeted manner by providing each phase module with at least one inductance and configuring the closed-loop control means to regulate a circulating current that flows through the phase modules.

BACKGROUND OF THE INVENTION Field of the Invention

The invention relates to a device for converting an electrical currentwith at least one phase module, which has an AC voltage connection andat least one DC voltage connection, a phase module branch being formedbetween each DC voltage connection and the AC voltage connection, andeach phase module branch having a series circuit comprising submodules,which each have an energy storage device and at least one powersemiconductor and with closed-loop control means for regulating thedevice.

Such a device is already known, for example, from the work by A.Lesnicar and R. Marquardt “An Innovative Modular Multilevel ConverterTopology Suitable for a Wide Power Range”, which appeared on Powertech2003. This paper discloses a power converter, which is intended to beconnected to an AC voltage system. The power converter has a phasemodule for each phase of the AC voltage system to be connected to it,each phase module having an AC voltage connection and two DC voltageconnections. Phase module branches extend between each DC voltageconnection and the AC voltage connection such that a so-called 6-pulsebridge circuit is provided. The module branches comprise a seriescircuit of submodules, which each comprise two disconnectable powersemiconductors, with which in each case inverse freewheeling diodes areconnected in parallel. The disconnectable power semiconductors and thefreewheeling diodes are connected in series, with a capacitor beingprovided in parallel with said series circuit. Said components of thesubmodules are wired to one another such that either the capacitorvoltage or the voltage zero drops across the two-pole output of eachsubmodule.

The disconnectable power semiconductors are controlled by means ofso-called pulse width modulation. The closed-loop control means forcontrolling the power semiconductors have measuring sensors fordetecting currents whilst obtaining current values. The current valuesare supplied to a central control unit, which has an input interface andan output interface. A modulator, i.e. a software routine, is providedbetween the input interface and the output interface. The modulator has,inter alia, a selector unit and a pulse width generator. The pulse widthgenerator generates the control signals for the individual submodules.The disconnectable power semiconductors are changed over from an onsetting, in which a current flow via the disconnectable powersemiconductors is made possible, to an off setting, in which a currentflow via the disconnectable power semiconductors is interrupted, bymeans of the control signals generated by the pulse width generator. Inthis case, each submodule has a submodule sensor for detecting a voltagedrop across the capacitor.

Further papers relating to the control method for a so-calledmulti-level power converter topology are those by R. Marquardt, A.Lesnicar, J. Hildinger “Modulares Stromrichterkonzept fürNetzkupplungsanwendung bei hohen Spannungen” [Modular power converterconcept for power supply system coupling application in the case of highvoltages], presented at the ETG technical conference in Bad Nauenheim,Germany 2002, by A. Lesnicar, R. Marquardt, “A new modular voltagesource inverter topology”, EPE' 03 Toulouse, France 2003 and by R.Marquardt, A. Lesnicar “New Concept for High Voltage Modular MultilevelConverter”, PESC 2004 Conference in Aachen, Germany.

The German patent application 10 2005 045 090.3, which is as yetunpublished, has disclosed a method for controlling a polyphase powerconverter with distributed energy storage devices. The disclosed devicelikewise has a multi-level power converter topology with phase modules,which have an AC voltage connection arranged symmetrically in the centerof each phase module and two DC voltage connections. Each phase modulecomprises two phase module branches, which extend between the AC voltageconnection and one of the DC voltage connections. In turn, each phasemodule branch comprises a series circuit of submodules, each submodulecomprising disconnectable power semiconductors and freewheeling diodesconnected back-to-back in parallel therewith. In addition, eachsubmodule has a unipolar capacitor. Closed-loop control means are usedfor regulating the power semiconductors, which closed-loop control meansare also designed to set branch currents which flow between the phasemodules. By controlling the branch currents, current oscillations, forexample, can be actively damped and operating points at lower outputfrequencies can be avoided. Furthermore, uniform loading of all of thedisconnectable semiconductor switches and symmetrization of veryasymmetrical voltages can be brought about.

The submodules of the phase modules generate in each case discreteoutput voltages, with the result that, given unequal voltage ratiosbetween the phase modules, circulating currents can be brought aboutbetween the individual phase modules. These circulating currents aredependent on the ratio of the voltages applied to the inductances withinthe current path, in addition to the switching frequency at which thepower semiconductors are switched. At low switching frequencies of below200 Hz, the circulating currents can barely be managed in terms ofregulation technology in the case of small inductances and cannot beavoided.

BRIEF SUMMARY OF THE INVENTION

The object of the invention is therefore to provide a device of the typementioned at the outset with which circulating currents can becontrolled and reduced in a targeted manner.

This object is achieved according to the invention by virtue of the factthat each phase module has at least one inductance, the closed-loopcontrol means being designed to regulate a circulating current, whichflows via the phase modules.

According to the invention, each phase module has at least oneinductance. The inductances are designed such that targeted regulationof the circulating currents is made possible by means of the closed-loopcontrol means. In other words, the inductances are matched to therespectively present conditions, such as the DC voltage applied, the ACvoltage applied or the like. The regulation predetermines desiredcirculating voltage setpoint values, which are applied during theregulation of the associated phase module branch as the setpoint value,for example other setpoint voltages of the phase module branch affected,and thus ensure a desired circulating current. In this case, theregulation advantageously has a current regulator and an associateddrive unit for each phase module branch. The current regulator isconnected to the submodules of the respective phase module branch onlyvia the drive unit, but not directly. In this case, the currentregulator generates, for example, a branch voltage setpoint value, whichis made available to the drive unit. The drive unit provides controlsignals, which are supplied to the disconnectable power semiconductorsof the submodules, with the result that a total voltage drop across thesubmodules of the associated phase module branch corresponds to thebranch setpoint voltage as precisely as possible. The application of thecirculating voltage setpoint values to other setpoint voltages of therespective phase module branch takes place by means of the currentregulator, which combines said setpoint values with one another inlinear fashion, i.e. by means of summation and/or subtraction. Theresult of this linear combination is branch voltage setpoint values,which are each associated with a phase module branch.

Since each phase module branch has an identical inductance, the requiredsymmetry in terms of regulation technology is provided.

Advantageously, each phase module branch is connected to the AC voltageconnection via an inductance. According to this expedient development,the AC voltage connection is arranged between two inductances.

In accordance with a development which is expedient in this regard, theinductances of the phase module are coupled to one another. The couplingincreases the total inductance, with the result that the individualinductances in terms of their values, i.e. their inductance, can becorrespondingly lowered. In this way, costs are saved. In other words,smaller inductors or coils can be used in the phase module. The totalinductance achieved by the coupling in addition affects only thecirculating currents and at best the DC components of the phase modulebranch currents. The inductance for AC-side phase currents is reduced bythe coupling of the inductances, however.

The coupling of the inductances can take place via air, via an iron coreor the like.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

Further advantages and configurations are the subject matter of thedescription below relating to exemplary embodiments of the inventionwith reference to the figures in the drawing, in which identicalreference symbols relate to functionally identical component parts andin which:

FIG. 1 shows an exemplary embodiment of a device according to theinvention in a schematic illustration,

FIG. 2 shows an equivalent circuit diagram of a submodule of a device asshown in FIG. 1,

FIG. 3 shows the device shown in FIG. 1 with coupled inductances,

FIG. 4 shows an enlarged illustration of the coupling of theinductances,

FIG. 5 shows the structure of the closed-loop control means of thedevice shown in FIG. 1, and

FIG. 6 shows the application of circulating voltage setpoint values toother setpoint values of the closed-loop control means.

DESCRIPTION OF THE INVENTION

FIG. 1 shows an exemplary embodiment of the device 1 according to theinvention which comprises three phase modules 2 a, 2 b and 2 c. Eachphase module 2 a, 2 b and 2 c is connected to a positive DC voltage linep and to a negative DC voltage line n, with the result that each phasemodule 2 a, 2 b, 2 c has two DC voltage connections. In addition, ineach case one AC voltage connection 3 ₁, 3 ₂ and 3 ₃ is provided foreach phase module 2 a, 2 b and 2 c. The AC voltage connections 3 ₁, 3 ₂and 3 ₃ are connected to a three-phase AC voltage system 5 via atransformer 4. The phase voltages U1, U2 and U3 drop across the phasesof the AC voltage system 5, with system currents In1, In2 and In3flowing. The AC-voltage-side phase current of each phase module isdenoted by I1, I2 and I3. The DC voltage current is I_(d). Phase modulebranches 6 p 1, 6 p 2 and 6 p 3 extend between each of the AC voltageconnections 3 ₁, 3 ₂ or 3 ₃ and the positive DC voltage line p. Thephase module branches 6 n 1, 6 n 2 and 6 n 3 are formed between each ACvoltage connection 3 ₁, 3 ₂, 3 ₃ and the negative DC voltage line n.Each phase module branch 6 p 1, 6 p 2, 6 p 3, 6 n 1, 6 n 2 and 6 n 3comprises a series circuit of submodules (not illustrated in detail inFIG. 1) and an inductance, which is denoted by L_(Kr) in FIG. 1.

FIG. 2 illustrates the series circuit of the submodules 7 and inparticular the design of the submodules by means of an electricalequivalent circuit diagram in more detail, with only the phase modulebranch 6 p 1 being singled out in FIG. 2. The rest of the phase modulebranches have an identical design, however. It can be seen that eachsubmodule 7 has two disconnectable power semiconductors T1 and T2connected in series. Disconnectable power semiconductors are, forexample, so-called IGBTs, GTOs, IGCTs or the like. They are known to aperson skilled in the art as such, with the result that a detailedillustration is not required at this juncture. A freewheeling diode D1,D2 is connected back-to-back in parallel with each disconnectable powersemiconductor T1, T2. A capacitor 8 is connected as the energy storagedevice in parallel with the series circuit of the disconnectable powersemiconductors T1, T2 or the freewheeling diodes D1 and D2. Eachcapacitor 8 is charged in unipolar fashion. Two voltage states can nowbe generated at the two-pole connection terminals X1 and X2 of eachsubmodule 7. If, for example, a drive signal is generated by a driveunit 9, with which drive signal the disconnectable power semiconductorT2 is changed over into its on setting, in which a current flow via thepower semiconductor T2 is made possible, the voltage drop across theterminals X1, X2 of the submodule 7 is zero. In this case, thedisconnectable power semiconductor T1 is in its off setting, in which acurrent flow via the disconnectable power semiconductor T1 isinterrupted. This prevents the discharge of the capacitor 8. If, on theother hand, the disconnectable power semiconductor T1 is changed over toits on setting, but the disconnectable power semiconductor T2 is changedover to its off setting, the full capacitor voltage Uc is present at theterminals X1, X2 of the submodule 7.

The exemplary embodiment of the device according to the invention shownin FIGS. 1 and 2 is also referred to as a so-called multi-level powerconverter. Such a multi-level power converter is suitable, for example,for driving electrical machines, such as motors or the like, forexample. Furthermore, such a multi-level power converter is alsosuitable for use in the sector of energy distribution and transmission.Thus, the device according to the invention is used, for example, as aback-to-back link, which comprises two power converters which areconnected to one another on the DC-voltage side, the power converterseach being connected to an AC voltage system. Such back-to-back linksare used for the exchange of energy between two energy distributionsystems, the energy distribution systems having, for example, adifferent frequency, phase angle, neutral-point connection or the like.Furthermore, applications in the field of wattless power compensation asso-called FACTS (Flexible AC Transmission Systems) come intoconsideration. High-voltage DC transmission over long distances is alsoconceivable with such multi-level power converters.

The inductances L_(Kr) are used for limiting the currents flowing viathe respective phase module and therefore for protecting thedisconnectable power semiconductors T1, T2 and the freewheeling diodesD1 and D2 of the submodules 7 from overcurrents. In the context of theinvention, however, the respective inductance is selected to be so highthat active regulation of the circulating currents which flow betweenthe phase modules is made possible. In the context of the invention,therefore, inductances are required which are higher than those whichare sufficient merely for protecting the power semiconductors.Furthermore, a symmetrical distribution of the inductances over thephase module branches with a view to regulation is advantageous.

FIG. 3 shows the device shown in FIG. 1, but with the inductances L_(Kr)of a phase module being coupled to one another. As a result of thiscoupling, the inductances may be lower than in the exemplary embodimentshown in FIG. 1 given the same rated voltages and the same useconditions. In other words, the coupling provides the possibility ofreducing the inductors or coils required for construction in terms oftheir physical size and the rest of their configuration. On the basis ofa coupling factor K for the magnetic coupling, the following results forthe effective inductance of a phase module branch in the circulatingcurrent direction L_(K):L _(K) =L _(L)(1+K),where L_(L) corresponds to the inductance of the sum of the individualinductances which are not coupled to one another. The phase modulebranch currents comprise, in addition to the circulating currents, DCcurrent components and phase currents I1, I2 and I3 flowing between theAC voltage connections 3 ₁, 3 ₂, 3 ₃ and the connected AC voltagesystem. An increased inductance results only for the DC components andthe circulating currents. The inductance L_(CONV) for the phase currentsI1, I2 and I3 is reduced, however, by the coupling in accordance withL _(CONV) =L _(L)(1−K).In this way, circulating currents can be reduced and can be supplied foractive regulation. The coupling can take place via air, but also via aniron core or the like. In the case of air-core inductors, couplingfactors of up to 20% can be produced. In addition to the damping of thecirculating currents, the coupled inductances also ensure improvedsplitting of the phase currents into identical components between thephase module branches of the same phase module.

FIG. 5 illustrates the structure of the closed-loop control means. Theclosed-loop control means comprise a current regulator 10 and driveunits 9 p 1, 9 p 2, 9 p 3 and 9 n 1 and 9 n 2 and 9 n 3. Each of thedrive units is associated with a phase module branch 6 p 1, 6 p 2, 6 p3, 6 n 1, 6 n 2 and 6 n 3, respectively. The drive unit 9 p 1 is, forexample, connected to each submodule 7 of the phase module branch 6 p 1and generates the control signals for the disconnectable powersemiconductors T1, T2. A submodule voltage sensor (not illustrated inthe figures) is provided in each submodule 7. The submodule voltagesensor is used for detecting the capacitor voltage drop across thecapacitor 8 as the energy storage device of the submodule 7 whilstobtaining a capacitor voltage value Uc. The capacitor voltage value Ucis made available to the respective drive unit, in this case 9 p 1. Thedrive unit 9 p 1 therefore obtains the capacitor voltage values of allof the submodules 7 of the phase module branch 6 p 1 associated with itand summates these values to obtain a branch energy actual value or inthis case branch voltage actual value UcΣp1, which likewise isassociated with the phase module branch 6 p 1. This branch voltageactual value UcΣp1 is supplied to the current regulator 10.

Moreover, the current regulator 10 is connected to various measuringsensors (not illustrated in the figures). Thus, current transformersarranged on the AC-voltage side of the phase modules 2 a, 2 b, 2 c areused to generate and supply phase current measured values I1, I2, I3 andcurrent transformers arranged at each phase module are used to generateand supply phase module branch currents Izwg and a current transformerarranged in the DC voltage circuit of the power converter is used toprovide DC current measured values Id. Voltage transformers of the ACsystem provide system voltage measured values U1, U2, U3 and DC voltagetransformers provide positive DC voltage measured values Udp andnegative DC voltage measured values Udn, the positive DC voltage valuesUdp corresponding to a DC voltage drop between the positive DC voltageconnection p and ground, and the negative DC voltage values Udncorresponding to a voltage drop between the negative DC voltageconnection and ground.

The current regulating unit 10 is also supplied setpoint values. In theexemplary embodiment shown in FIG. 5, the regulating unit 10 is suppliedan active current setpoint value Ipref and a wattless current setpointvalue Iqref. In addition, a DC voltage setpoint value Udref is appliedto the input of the current regulating unit 10. Instead of a DC voltagesetpoint value Udref, the use of a DC setpoint value Idref is alsopossible in the context of the invention.

The setpoint values Ipref, Iqref and Udref and said measured valuesinteract with one another when using different regulators, with a branchvoltage setpoint value Up1ref, Up2ref, Up3ref, Un1ref, Un2ref, Un3refbeing generated for each drive unit 9 p 1, 9 p 2, 9 p 3, 9 n 1, 9 n 2and 9 n 3. Each drive unit 9 generates control signals for thesubmodules 7 associated with it, with the result that the voltage dropUp1, Up2, Up3, Un1, Un2, Un3 across the series circuit of the submodulescorresponds to the respective branch voltage setpoint value Up1ref,Up2ref, Up3ref, Un1ref, Un2ref, Un3ref as far as possible.

The current regulator 10 forms suitable branch voltage setpoint valuesUp1ref, Up2ref, Up3ref, Un1ref, Un2ref, Un3ref from its input values.

FIG. 6 shows that, for example, the branch voltage setpoint value Uprefis calculated by linear combination of a system phase voltage setpointvalue Unetz1, a branch voltage intermediate setpoint value Uzwgp1, a DCvoltage setpoint value Udc, a symmetrizing voltage setpoint value Uasymand a balancing voltage setpoint value Ubalp1. This takes place for eachof the phase module branches 6 p 1, 6 p 2, 6 p 3, 6 n 1, 6 n 2, 6 n 3independently of one another. The circulating currents can be set in atargeted manner using the branch voltage intermediate setpoint valuesUzwg in conjunction with the set branch inductances. The balancingvoltage setpoint values Ubal are also used for compensating forasymmetries as regards the energies stored in the phase module branches6 p 1, 6 p 2, 6 p 3, 6 n 1, 6 n 2, 6 n 3.

1. A device for converting an electrical current, the device comprising:at least one phase module each having an AC voltage connection, at leastone DC voltage connection, at least one phase module branch respectivelydisposed between each said at least one DC voltage connection and saidAC voltage connection, and at least one inductance; each said at leastone phase module branch having a series circuit including submoduleseach having an energy storage device, and at least one powersemiconductor; and a closed-loop control device for regulating thedevice, said closed-loop control device regulating a circulating currentflowing between each said at least one phase module branch on a closedcurrent path.
 2. The device according to claim 1, wherein each said atleast one phase module branch is connected to said AC voltage connectionthrough said respective at least one inductance.
 3. The device accordingto claim 1, wherein said at least one inductance of a phase module is aplurality of inductances coupled to one another.
 4. The device accordingto claim 3, wherein said inductances are coupled to one another throughair.
 5. The device according to claim 3, which further comprises an ironcore through which said inductances are coupled to one another.